Is it possible to dim LED christmas lights?

No, it won't LEDs have a minimum operating voltage, and if the available voltage drops below that you don't get any light. You either run them from an adjustable constant current source, or Pulse Width Modulate the supply to adjust the brightness.

--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

by

Fair point but I assume there must be some form of current limiting in the exisiting strings. Surely these are not just connected across the rectified mains supply? If the current is limited by a resistor then the brightness could be controlled by increasing its value.

Bob in UK

Whats problem with dimming? Simply get rheostat and use in serries. There has to be even nos of rows as LEDs are connected reverse in one row and forward in another. So assuming a small resistor in each row, the rheostat will control current till it becomes very small and LEDs are off. We can asume that each row has 60LEDs and current is 5mA(10 mA halfwave) so resistor in each row is 1200 ohms. & toatal 16 rows. If one connects 500 ohm 10W rheostat or potentiometer then intensity can be controlled.

It is not posible to dim LEDs with PWM as peak current in them remains same and due to persistance of vision intensity appears same. Though current may be less. Only constant current regulation can do the job.

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That's not true.

Assume an LED with 20 mA through it all the time.  It will exhibit a
certain brightness.  Now, still with 20 mA through it, PWM it so
it's only on for half the time but with a PRF high enough so that
you can't detect any flicker.  It will appear to be half as bright
as before.

In article , psdayama wrote in part:

The myth persists, and this is as bad as I ever heard it!

Meanwhile, I have more on:

about pulsing LEDs to increase ratio of perceived output to average current (while appearing to glow continuously), when this works, when this does not, and how little human vision has to do with it when it does work!

PWM is a common way to dim LEDs in RGB displays. Many manufacturers recommend PWM because LED output and color as a function of varying current can vary from one unit to another even when different units perform the same at "characterization current", and some LEDs normally have color varying with current.

- Don Klipstein ( snipped-for-privacy@misty.com)

YES IT IS TRUE!!!!! First U dont know anything about LEDs. At 20mA normal LEDs will blow up soon. But U can get the same intensity of LED with oulsing 20mA current with

25% duty cycle. Thats called PWM. It is not PWM of voltage which will not work. But there are ckts which can be current controllers. Now most of the LCD displays have also background LEDs pulsed current so they consume less power but dispaly is same. Above PRF of 100 persistance of vision works which shows LED bright as if consuming high current. Pls go to some wikipedia or some sites of LED manufacturers and U will know. Ignorance is greatest bliss !!!!

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No, it isn't.

Oops...

Joules per second.

--
JF

20mA is fine for normal leds.

BULLSHIT! try it.

that's just nonsense.

resistors work great for LEDs.

Bye. Jasen

It is the paradox of vision. Ur eye cant detect fast changing intensities and due to persistance of vision it appears as bright as if 100% lumens out. As per Ur theory the whole new led lamps will go out of business as 3W high efficiency LED shines as if 40W GSL lamp. It doesnt consume that much power to give that Lumens. Shallow waters make biggest noise!

--- Nope. Your eye integrates the light energy that comes into it, so if you see four times as many photons in 1/4 of the time that the same number of photons entered continuously your eye would see both sources as equally bright.

Don't believe me? Try it.

Get two LEDs that are equally bright with, say, 2mA going through them from a DC source. Now, disconnect one of them and assemble some equipment, like this: (View in Courier)

+------+ 2mA-->

| DC +|--[1000]---+ |SUPPLY| | | -|--[]---+---|SCOPE| | GEN | | +--+--+ +--+--+ [1000] | | | | +---------------+------+ | GND

Adjust the pulse generator's output amplitude so that it's below the illumination threshold of the LED, its waveform for 25% duty cycle, and its rep rate high enough so that you can't see flicker. (I used

100Hz and 1000 Hz...)

Now, adjust the generator's output amplitude, watching both LEDs until they appear to be the same brightness, then read the peak amplitude of the waveform on the scope and multiply it by the resistance of R2. You'll find it's four times the current in the other LED.

I chose to use high-efficiency red LEDs (HLMP4700) but you should get the same results, should you choose to run the experiment, with whatever LEDs you choose.

---

--- That's efficiency, and has nothing to do with what we've been talking about at the level at which we've been talking.

---

--- Then you must be very nearly dry...

-- JF

Sure they can: The light output is proportional to the current through them

Easy way to dim LED christmas lights... Plug them into an X10 outlet that is turned OFF. When an X10 outlet is turned off it is still putting out about 60 volts (I checked over a dozen different types of X10 outlet, in-wall type as well as plug-in type, and they all put out around 60 volts). Might be a little TOO dim though.

I'm try> DeanB wrote:

power consumed by yourlightsand,

these.http://froogle.google.com/froogle?scoring=p&price=between&price1=1&q=...

Try it with a load that is more meaningful than just a 10Mohm voltmeter.